The present invention pertains in general to ion-selective electrodes and in particular to planar, ion-selective electrodes having non-metallic conductor patterns.
When placed in contact with a solution, ion-selective electrodes provide an electrical output which is a function of the concentration of a particular ion in the solution. In such electrodes an output potential ("Y") is measured between a "sensing element, responsive to the concentration of the particular ion, and a "reference element," held at a constant potential, Y may be plotted against the base 10 logarithm of the concentration of the ion ("X") as a straight line having a slope ("M") and y-axis intercept ("B") as expressed in the Nernst equation: EQU Y=M(log.sub.10 X)+B
Ion-selective electrodes conventionally have an internal reference element of Ag/AgCl immersed in a solution or gel of chloride ion. The chloride ion solution or gel holds the reference element at a constant potential, providing that the chloride concentration and thermodynamic parameters, such as temperature and pressure, are held constant. An ion-selective glass or membrane sensing element is placed in contact with the solution or gel to form an interface between test solution and this internal filling solution. However, this conventional design is complex to manufacture and difficult to miniaturize.
Two alternative configurations, chemical field effect transistors (ChemFETs) and coated wire ion-selective electrodes, have a simplified processing design. Nevertheless, these configurations do not reproduce the low drift characteristics of the conventional design. Also, neither configuration is an improvement upon the interface (i.e. membrane-gel/liquid interface) which is inherently better in the conventional design. The conventional ion-selective electrode approach, which employs an internal reference element bathed in an ionic solution or gel, is still the design of choice for most ion-selective electrode applications.
Another alternative configuration employs a graphite rod rendered hydrophobic by such materials as oil, paraffin, a silanizing agent or Teflon.RTM., a calomel paste rubbed into the graphite surface as a reference element, and an ion-sensitive membrane. Ruzicka et al., U.S. Pat. No. 3,926,764; and Brown et al., U.S. Pat. No. 4,431,508. The design is difficult to process or miniaturize due to a reliance upon the use of carbon rods.
Yet another ion-selective electrode is formed by mounting a cylindrical graphite plug having a first flat surface co-planar with the surface of an insulating substrate and having a second flat surface with a conductive layer which connects to the graphite plug, which passes through the substrate. Knudson et al., U.S. Pat. No. 4,549,951. The first surface of the graphite is completely covered by an electroactive membrane. This electrode is also difficult to miniaturize due to the use of a carbon rod.
In Israel Patent Application No. 33483, assigned to Hydronautics Israel Limited, a conductor covered by an ion-specific membrane at the end of an ion-specific probe may be formed of a compacted graphite powder. However, the use of a compacted powder does not suggest a readily miniaturized structure nor does any electrode in which a metallic conductor positioned directly beneath a membrane/carbon interface, such as the electrodes found in Hydronautics Israel and Knudson et al., provide a solution to the problem of leakage leading to corrosion of the metal conductor and posioning or drift resulting from such corrosion.
In an attempt at miniaturization of a graphite-containing ion-selective electrode, a plug of conductive carbon, in particular a plug of compressed graphite is embedded in a first surface of an electrically insulating cap and is separated from a solution to be tested by an hydrophobic material including a ligand capable of selective metal complexation, such as ion selective antibiotics, macrocyclics, open chain neutral or ionic ligands, certain inorganic compounds and mixtures thereof. Hawkins, U.S. Pat. No. 4,276,141. However, the need for placement of a plug directly over and of an electrical conductor passing through the cap limits both the ease with which such a device may be manufactured and the variety of configurations attainable, e.g. the number and type of electrodes per unit of surface area.
In order to achieve greater flexibility and to promote ease of miniaturization, a carbon layer of finely divided particles of carbon uniformly dispersed in a matrix of an organic polymer may be coated on a conductive layer printed on a ceramic substrate. Pace, U.S. Pat. No. 4,454,007. Although the metal conductors of Pace are somewhat shielded from contact with an analyte by a combination of a carbon layer intersolubilized with an exposed ionophoric ion-selective layer and by an insulating layer, moisture may penetrate around the ionophoric and graphite layers to directly contact and corrode the metal conductors beneath them, a corrosion problem shared with other printed electrodes. See, e.g. Hamblen et al., U.S. Pat. No. 4,053,381; Kratochvil et al., U.S. Pat. No. 4,449,011; Battaglia et al., U.S. Pat. No. 4,214,968; Kim et al., U.S. Pat. No. 4,272,328; Afromowitz et al., U.S. Pat. No. 4,133,735 Gottermeier, U.S. Pat. No. 4,273,639; Johnson et al., U.S. Pat. No. 4,020,830; Pace, U.S. Pat. No. 4,225,410; Paul et al., U.S. Pat. No. 4,184,936; Kitajuna et al., U.S. Pat. No. 4,528,085; Ho et al., U.S. Pat. No. 4,393,130; Guckel, U.S. Pat. No. 4,180,771; Blackburn, U.S. Pat. No. 4,486,292; and Blackburn, U.S. Pat. No. 4,456,522.